Trench

About: Trench is a research topic. Over the lifetime, 26800 publications have been published within this topic receiving 298633 citations.

Back-arc opening and the mode of subduction

Abstract: Trench-arc systems (subduction zones) can be classified into two types depending on whether or not actively opening back-arc basins are associated with them. This suggests that subduction of an oceanic plate is not a sufficient condition for back-arc opening, though it may be necessary one. Mechanisms that cause the distinction between the two types have been investigated. Earthquake studies suggest that there is a significant difference in the mode of plate motion at interplate boundaries between the two types of trench-arc systems. Extreme cases are Chile, where plate motion is seismic, and the Marianas arc, where it is aseismic. This difference seems to indicate that the stress state in the back-arc area differs between the two types: compression in the Chilean type and tension in the Marianas type. This difference in the stress state is also manifested in other tectonic features, such as topography, gravity, volcanic activity, and crustal movement. Two possible mechanisms for the difference between the two types are suggested: (1) The nature of the contact zone between upper and lower plates changes from tight coupling (Chile) to decoupling (the Marianas) through the evolutionary process of subduction. The decoupling results in an oceanward retreat of the trench and back-arc opening. (2) The downgoing slab is anchored to the mantle, so that the position of a trench is also fixed with respect to the mantle. Since the motion in the mantle is slow compared to that of surface plates, it is the motion of the landward plate which controls the opening and nonopening of back-arcs.

Formation Mechanism and Properties of Electrochemically Etched Trenches in n‐Type Silicon

Abstract: Three‐dimensional structures in silicon are increasingly coming into use for the fabrication of mechanical and electrical devices. The fabrication of deep trenches is one of the most important problems in VLSI (very large scale integration) technology. In this study the spontaneous trench formation in n‐type silicon immersed in hydrofluoric acid under anodic bias is demonstrated and the resulting microstructures are characterized. Trenches with arbitrary cross sections and high aspect ratios for microelectronic (e.g., 42 μm depth and 0.6 μm diam) and for power component application (e.g., 60 μm depth and 10 μm diam) have been produced by a standard masking technique. The trench formation is explained by a model which takes into account the conditions of the space charge region the minority carrier current and the crystal orientation. A passivating sidewall layer is not needed in this model. The dimensions and the shape of anodically etched trenches can be varied over a wide range by adjusting the critical parameters.

Oblique plate convergence, slip vectors, and forearc deformation

Abstract: Slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis, suggesting that oblique convergence is taken up by partial decoupling. Decoupling means that a component of arc-parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench. Partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike-slip faults that are parallel to the trench and to each other and, starting with a force equilibrium condition, a relationship between the obliquity and the earthquake slip vector orientation is derived. Assuming that either fault slips when shear stress on it reaches a yield stress, oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle. For obliquity at or greater than this angle the stress on the strike-slip fault is large enough to start it slipping, and when both faults are active, the arc-parallel motion of the forearc deflects the slip vector back toward the trench-normal. If we assume that continued slip on either fault occurs at constant stress (but the two faults can be at different stresses), the slip vector will maintain a constant angle relative to the trench-normal even when obliquity is larger than the critical angle. This limiting angle of the slip vector, called ψmax (measured relative to the trench-normal), is simply the arcsine of the ratio of the shear forces resisting slip on the strike-slip and thrust faults. A consequence is that when the obliquity exceeds ψmax the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates. Slip vectors at the Java trench southwest of Sumatra show the relationship clearly with ψmax =20°±5°, while slip vectors at the Aleutian trench show the relationship less clearly with ψmax=25° to 45°. The greater angle at the Aleutian trench suggests that the upper plate is stronger in the Aleutian arc (relative to the thrust fault) than in the Sumatran arc, consistent with the Sumatran arc being continental and having a well-developed strike-slip fault while the Aleutian arc is oceanic and without a clear transcurrent fault. Slip vectors at the Philippine trench which, like Sumatra, has a large strike-slip fault inboard of it, tend to stay within 25° of the trench-normal when obliquity is as large as 50°. If obliquity exceeds ψmax and continues to increase along a subduction zone, the rate of motion of the forearc relative to the upper plate will vary with obliquity, in which case the forearc sliver should extend or contract parallel to the arc. From the geometry of modern island arcs, arc-parallel extension should be the more common and has been hypothesized for both Sumatra and the Aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations. From estimates of ψmax and the arc-parallel gradients in obliquity, arc-parallel strain rates are estimated to be 1 to 3×10−8/yr for the Sumatran forearc, 2 to 6×10−8/yr for the Aleutian forearc, and 0.3 to 3×10−8/yr for the Philippine forearc. Oblique convergence and subsequent arc-parallel extension, if accompanied by crustal thinning, may provide an important yet little appreciated mechanism for bringing high-grade metamorphic rocks to the surface of subduction complexes.

Threshold relations in subsurface stormflow: 2. The fill and spill hypothesis

Abstract: Analysis of subsurface stormflow from 147 storms at the 20 m long trench in the Panola Mountain Research Watershed by Tromp-van Meerveld and McDonnell (2006a) showed that there was a distinct 55 mm precipitation threshold for significant subsurface stormflow production. This second paper in the series investigates the processes responsible for this threshold response. We installed a dense spatial array of maximum rise crest stage gauges and recording wells on the hillslope and studied the temporal and spatial patterns of transient saturation at the soil-bedrock interface and its relation to subsurface stormflow measured at the trench face. Results show that while transient groundwater developed on parts of the hillslope during events smaller than 55 mm, it was not until more than 55 mm of rain fell before bedrock depressions on the hillslope were filled, water spilled over microtopographic relief in the bedrock surface, and the subsurface saturated areas became connected to the trench. When connectivity was achieved, the instantaneous subsurface stormflow rate increased more than fivefold compared to before the subsurface saturated areas were connected to the trench face. Total subsurface stormflow was more than 75 times larger when connectivity was achieved compared to when connectivity was not achieved. The fill and spill hypothesis presented in this paper is a process explanation for the observed threshold behavior of Tromp-van Meerveld and McDonnell (2006a), thereby linking patterns and processes.